WASHINGTON — Navy jets and drones pounded ISIS rebels Saturday in a series of four airstrikes, according to U. S. Central command.

The first attack occurred around 11:20 a.m. ET. Surveillance aircraft observed an armored personnel carrier firing at civilian targets. It was destroyed as were several other vehicles about 20 minutes later in two additional strikes.

A fourth airstrike occurred at 3 p.m. ET, destroying an armored vehicle, according to Central Command, which oversees military operations in the Middle East.

In a statement Saturday, U.S. Central Command said the attacks were "to defend Yazidi civilians being indiscriminately attacked" by militants.

While the Islamic State's heavy weapons and armor have made it a potent fighting force against Iraq's military, they also make the militants vulnerable to newly authorized American airstrikes.

Unlike insurgents who use hit-and-run tactics, the Islamic State is operating more as a conventional army, often moving through open terrain where they can be easily struck from the air.

"It inherently gives you a more visible footprint," said Jeff White, a former Defense Intelligence Agency official now at the Washington Institute for Near East Policy. White cautioned, however, that the militants do not have the organization and hierarchy of a regular military unit with a clear-cut structure.

The United States has been flying up to 50 surveillance and reconnaissance missions a day over Iraq in recent weeks, providing the American military with a clear picture of the enemy.

Brett McGurk, a State Department official, said recently the Islamic State functions more as a "full-blown army" than a typical terrorist organization. The militants are operating in large groups, with tanks, artillery and other heavy weapons, much of it U.S. equipment captured from Iraq's army.

By contrast, the al-Qaeda-linked insurgents the United States fought in Iraq after the U.S.-led invasion in 2003 were equipped only with light arms and operated in small groups.

In Afghanistan, American forces have faced an enemy that operates in small groups without heavy equipment, allowing them to easily hide in the mountains.

Kurds and Yazidi take part in a protest rally in Frankfurt, Germany, on Aug. 9, 2014.(Photo: Boris Roessler, European Pressphoto Agency)

Since President Obama authorized the strikes Thursday, U.S. aircraft have hit convoys, mortars and artillery manned by militants.

On Saturday, Obama said there was no timeline for ending the strikes.

"So far, these strikes have successfully destroyed arms and equipment that ISIL terrorists could have used against Irbil," Obama said, referring to the militant group by its acronym.

Obama said he authorized the airstrikes in order to protect Americans and support a humanitarian mission to assist Yazidis, a religious minority that fled their homes and were trapped in the mountains.

Islamic State militants had driven the Yazidis from their homes in Ninjar and were beginning to threaten Irbil, the capital of the Kurdish region and home to a U.S. Consulate and a joint U.S.-Iraqi military operations center.

The Kurdish militia, called the peshmerga, is a well-disciplined force, but prior to the airstrikes they were struggling to defend towns and villages along the border of the Kurdish region.

They faced a formidable enemy in the Islamic State, which captured tanks, armored vehicles and other weapons and equipment, much of it supplied by the United States, when they attacked Mosul in June. Four Iraqi divisions collapsed at the time, leaving much of their equipment and arms behind.

The Pentagon's airstrikes against Islamic militants in northern Iraq appear to have weakened Islamic State offenses around Irbil. President Obama authorized airstrikes to protect American personnel in Irbil and humanitarian drops to aid refugees who had fled to the Sinjar Mountains to escape Islamic State death threats.